The invention disclosed relates to a circuit for monitoring and controlling the flow of hot air in equipment for roasting coffee, nuts and similar edible commodities.
Conventionally, the process of roasting a given quantity of coffee is implemented in two distinct steps. First, the coffee is subjected to a pre-roast application of heat to lower its moisture content, selecting temperature values such as are able to ensure a well-balanced color of the beans; to best advantage, the temperature adopted in pre-roasting the raw commodity will be of the order of 150.degree.-160.degree. C.
The successive second step involves subjecting the pre-heated product to roast temperature proper, generally 220.degree.-230.degree. C.
Needless to say, the temperatures effectively used can vary considerably according to the different qualities of roast it is wished to produce. The need for a batch of coffee to be roasted in two distinct and successive steps is dictated by the fact that the single beans will never be ripened to an identical degree. With former methods, by which the full roast was effected in a single operation, attempts to produce the requisite color and flavor on particularly unripe beans led to pyrolisis in the riper beans, with clear negative consequences for the end-product as a whole.
In a previous application for Italian patent filed under No. 3450 A/82 by the same applicant, a system is disclosed whereby the complete coffee roasting cycle can be effected using special equipment that comprises, amongst other components, a first and a second drum of conventional embodiment; these drums are of substantially equal capacity, and will be positioned to best advantage one above the other, or at all events, at dissimilar heights. Thus, in implementing the two steps described above, the pre-roasted coffee beans, divested of moisture in the first drum, can be transferred through suitable ducts into the roasting drum proper, swiftly and at the opportune moment.
The two drums are provided with a respective inlet and outlet to which respective hot air ducts are connected, the hot air being produced by a suitable generator and circulated through the ducts by a fan unit.
The volume of air caused to flow through the inlet and outlet ducts is regulated by dampers programmed to open and shut at selected moments that will vary according to the quantity and quality of the blend and the duration of the roast, and can be operated by manual or automatic means to suit requirements. Hitherto, the major problem besetting the use of equipment of the type in question has been one of arriving at a comprehesive and precise integration of two parameters governing the entire roast cycle, namely: the temperature level generated internally of the two drums, and the time required to reach the selected level. More exactly, the temperature rise programmed for the first drum must be produced in the same interval of time as that programmed for the second drum, by no means a simple matter when one considers the difference between the two levels and the many and various thermal inertia components traceable to the coffee itself.
Where these two temperature rises are faultlessly timed, one achieves a continuous and regular cycle whereby one charge of full roast is emptied from the second drum, and at the same moment, another charge of pre-roast is transferred into the second drum from the first. Additionally, and of great importance, the single roast cycle effected on each charge of coffee will ensure the same pre-roast and roast temperatures as those of the previous cycles, such that the entire batch of coffee turned out during the full program of cycles implemented will be uniformly roasted and colored, giving optimum blends.
Such results have remained unobtainable thus far, in practice, by reason of the fact that the dampers utilized to control the hot air ducts to and from the first and second drums were adjusted by degrees only, opening and closing in discrete steps. The roasting cycle effected on a given quantity of coffee can now be considered in general terms, departing for convenience's sake from the roast proper; this occurs in the second drum, in which a temperature rise approximately between 155.degree.-160.degree. C. and 220.degree.-230.degree. C. must be produced.
Reference may be made here to the graph of FIG. 2a, which shows the variation in temperature T versus time t that occurs internally of the second drum, and to that of FIG. 2b which shows the corresponding variation in the angle .alpha. of the damper, also versus time t.
It will be seen that the damper is fully open at the outset: .alpha.=90.degree.. A predetermined quantity of hot air (calculated in calories) is directed into the second drum by the fan, sufficient to raise the temperature to an initial level T1 within a given interval of time t1; this is indicated in FIG. 2a by a straight line a, which reflects a substantially linear relationship between T and t, and an angular coefficient denoted .delta..
On arrival at T1, the damper will throttle down by some 45.degree., whereupon the increase in temperature T is slowed up and the set point T2 reached at t2; this further rise is reflected by the straight line denoted b, which has an angluar coefficient of .delta./2. The damper closes completely on arrival at T2, at which point the temperature of the coffee is at a level of the order such as to induce spontaneous ignition; accordingly, the temperature T should now rise a few degrees to Ts (phantom line c), reaching a level coinciding with the end-of-cycle mark ts on the time axis, at which discharge is to programmed to occur, and the roast is complete.
The part of the curve denoted c in FIG. 2a reflects the critical point of the roast part of the cycle in the second drum, but is indicative only, as the effective configuration depends on thermal inertia in the coffee (moisture, oils, grease etc.).
The interval of time embraced by the part of the curve c in question is that between the moment of the damper being shut and the moment that the drum begins emptying. In the majority of instances, it happens that the roast is not regulated by the flow of hot air, and the coffee fails to arrive at the spontaneous ignition temperature which produces the rise from set point T2 to discharge Ts; rather, the temperature tends to fall instead of rising (see d in FIG. 2a), and a thermoregulator with its probe located internally of the roasting drum will cut in to re-open the damper, say to 45.degree., for a further interval from t3 to t4, thereby ensuring that the temperature rises to the prescribed level Ts. The result of such an occurence is that the roast time become extended, and the second drum is not ready to receive the pre-roasted beans from the first drum.
Thus it happens that the pre-roast is forced to remain longer in the first drum also (see FIG. 1a), and even in the unlikely event that the critical stage reflected by curves c and d is avoided (the same criteria apply as for FIG. 2a described above), the temperature of the coffee will fall on arrival at Ts following movement of the damper to the off position, as indicated by the curve denoted f.
Accordingly, a thermoregulator monitoring the first drum will cut in at t5 and re-open the damper, say to 45.degree., for a further duration t6, thereby enabling the discharge level Ts to be restored; the second drum will in fact have emptied by this time and the pre-roast can be transferred.
In such a situation, the beans in the first drum are invested with more heat than is opportune, and drop into the second drum pre-roasted too highly; thus, the roast step implemented in the second drum must be cut short in order to ensure that the beans will emerge roasted and colored to the same degree as previous charges.
With the roast now cut short, the second drum will be ready to receive a further charge of pre-roast from the first drum before the pre-roasting step is in fact terminated, and should this occur, i.e. in the event that a charge of pre-roast is transferred from the first drum to the second drum short of the temperature level prescribed, one will be left with an under-roasted, lower temperature product at the end of the roast cycle, and the roast step must be prolonged in order to compensate; thus, the pre-roast is forced once again to remain in the first drum (remembering that the cycle is geared to the roast time-lapse in the second drum), and there is a recurrence of all the negative factors examined above. The end result is that one has a succession of fluctuations in roast and pre-roast time-lapses, which become increasingly marked, and the product necessarily suffers through the impossiblility of obtaining a regular roast, and in particular, a regular color, at each cycle.
More exactly, assuming the number of calories put into the drum as par, a longer roast will allow heat to penetrate deeper into the bean, whereas with a shorter roast, the bean will scorch on the surface and remain raw inside.
Accordingly, the object of the invention is one of overcoming the drawbacks described above.